In general, it is considered that peptic ulcers would be caused when an imbalance occurred between attack factors such as gastric juice, pepsin, etc., and mucus, bicarbonate ion secretion phase, blood flow or the like. Their development is specific to stomach and duodenum. Medical drug therapy for these peptic ulcers has been shifted from therapy laying stress on antiacidic agents or anticholinergic agents to therapy attaching importance to histamine H.sub.2 receptor antagonists showing a strong acid secretion inhibitory activity by blocking wall cell receptors. However, it is reported in Marks, I. N. et al., "Ulcer healing and relapse rate after initial treatment with cimetidine or sucralfate", J. Clin. Gastroent., 3 (Suppl. 2), 163-165 (1981) and Martin, D. F. et al., "Difference in relapse rates of duodenal ulcer after healing with cimetidine or tripotassium dicitrato bismuthate", Lancet, I, 7-10 (1981) that when administration of the histamine H.sub.2 receptor or antagonists was discontinued, relapse of ulcer was noted with high frequency.
In recent years, based on a new finding in the acid secrection mechanism in gastric wall cells and mucus protecting mechanism, antiulcer agents which inhibit [H.sup.+ -K.sup.+ ] adenosine triphosphatase (ATPase) participating in the final stage during the course of acid secretion in wall cells and which prevent secretion of gastic juice have been proposed, for example, in JP-B-60-34956 (The term "JP-B" as used herein means an "examined Japanese patent publication").
On the other hand, turning to imidazo[2,1-b ]benzothiazole derivatives, they are already known and recited in publications, for example, as drugs for treatment of diabetes (JP-A-52-83586 (The term "JP-A" as used herein means an "unexamined published Japanese patent application")), as immune function regulators (JP B-62-48672, JP-A-56-68685, JP-A-56-71096, JP-A-56-138196, JP-A-57-40492, JP-A-57-149288, JP-A-58-109491, JP-A-59-78194, etc.), as 8-receptor blockers (JP-A-60-258184) and the like. However, no specific disclosure or even suggestion has been found on medical application as antiulcer compositions.
The present inventors have synthesized many analogous compounds using known benzothiazole compounds as starting materials and made extensive investigations on these compounds. As a result, it has been found that while imidazo[2,1-b]benzothiazole compounds according to the present invention have a chemical structure different from known drugs showing an antiulcer activity, the compounds exhibit an excellent antiulcer activity. The present invention has been achieved based on such findings.